1. Field of the Invention
The present invention relates to methods of inhibiting the precipitation of metal ions from aqueous solutions, and to the use of certain phosphonomethylene amino carboxylates in such scale inhibition methods.
2. Description of Prior Art
Most commercial water contains iron and alkaline earth metal cations, such as calcium, barium, magnesium, etc., and several anions such as hydroxide, bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products under the conditions of use, precipitates form. For example, when the concentrations of calcium ion and sulfate ion exceed the solubility of the calcium sulfate, a solid phase of calcium sulfate will form. Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on the surfaces of the water carrying system, they form scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. The presence of this scale is an expensive problem in many industrial water systems (e.g., boilers, cooling towers, evaporators, etc.), oilwells, and the like, causing delays and shutdowns for cleaning and removal. The prior art has not dealt with the preparation or use of phosphonomethylene amino C.sub.3-15 alkylene carboxylates or aralkylene carboxylates which are the subject of the present invention. These carboxylates are effective metal ion sequestrants and are useful in water treatment for the inhibition of scale formation in aqueous systems, including scale inhibition by "threshold" quantities thereof.